In conventional electrostatographic imaging, electrostatic latent images are formed on a surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed by radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print.
Although electrostatographic equipment is used worldwide, it possesses a significant disadvantage in that the energy consumption is quite high. Thus, equipment with lower power consumption has been designed. Toners that function in the lower power consumption equipment, or that which meets the need for higher print speeds, are generally designed to have low glass transition temperatures (Tg's) of for example about 55° C. to about 65° C. However, an image defect known as document offset (or “blocking”) can occur at temperatures as low as about 54° C. to as high as about 70° C. or more when the toner begins to flow. Thus, low-melt toners often have a significant document offset problem.
In one situation, the document offset problem can be evident in the electrostatographic printing device and process itself. For example, at low glass transition temperatures of about 54° C. to about 65° C. and added pressure, such as typically occur where several reams of paper are located in the output tray of a printing machine, document offset in the printed papers can occur. This document offset can be in the form of toner sticking to the paper of the sheet above it or, in the case of duplex printing, toner sticking to toner on the sheet above it. The result is two sheets of printer paper that have to be pulled apart or, in the worst case, toner on one sheet pulls off either toner or paper fibers from the sheet above or below it, resulting in less desirable print quality. Similar document offset problems can also occur after the printing process is complete, such as during the lifetime of the printed document.
Document offset problems can be exacerbated when the printed items may be subjected to higher than normal environmental conditions. Thus, for example, a printed sheet of paper that is expected to stay within an office or home environment (such as near room temperature of about 20° C. to about 25° C.) may not exhibit document offset. However, a printed sheet of paper that is expected to be subjected to higher temperatures, such as documents kept in the glove compartment or passenger compartment of an automobile (where temperatures can regularly exceed about 40 to about 60° C.) may exhibit substantial document offset. For example, one standard for such printed materials as automobile manuals requires that the printed material survives a temperature of 70° C. for four hours.
Known methods of reducing document offset include adding wax to the toner and applying an overprint coating to the substrate. However, as described above, known overprint coatings fail to adequately protect xerographic prints and fail to reduce document offset. In addition, known coating formulations fail to prevent the formation of hairline cracks on the print surface in response to thermal expansion of the toner, which creates an undesirable appearance. This is a particularly important issue for automobile manuals, book covers, etc., which require the prints therein to survive high temperatures for hours at a time, yet retain a neat appearance.
Accordingly, a need exists for a protective composition that provides overprint coating properties including, but not limited to, thermal and light stability and smear resistance, co-efficient of friction (slip), abrasion resistance, particularly in commercial print applications. The protective composition can be applied, for example, to a printed image formed by electrostatographic imaging methods, ink jet methods, or the like. More specifically, a need exists for an overprint coating that has the ability to wet over silicone fuser oil (generally found on xerographic substrates), permit overwriting, reduce or prevent thermal cracking, reduce or prevent document offset, and protect an image from sun, heat, etc. The compositions and processes of the present disclosure, wherein a xerographic print is coated with a radiation curable overprint composition, satisfy this need.